A task force convened by the Obama administration issued the most detailed study yet of blight in Detroit on Tuesday and recommended that the city spend at least $850 million to quickly tear down about 40,000 dilapidated buildings, demolish or restore tens of thousands more, and clear thousands of trash-packed lots.

It also said that the hulking remains of factories that dot Detroit, crumbling reminders of the city's manufacturing prowess, must be salvaged or demolished, which could cost as much as $1 billion more.

If carried out, the recommendations by the Detroit Blight Removal Task Force would drastically alter the face of the nation's largest bankrupt city. They would also cost significantly more than the approximately $450 million that the city already plans to spend on blight, raising questions about the feasibility of the vast cleanup effort, which is part of its larger campaign to emerge from bankruptcy by fall and begin remaking itself.

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The blight study, which is perhaps the most elaborate survey of decay conducted in any large America city, found that 30 percent of buildings, or 78,506 of them, scattered across the city's 139 square miles, are dilapidated or heading that way. It found that 114,000 parcels -- about 30 percent of the city's total -- are vacant. And it found that more than 90 percent of publicly held parcels are blighted.